我们先做下配置：
R1:interface Serial1/0
ip address 13.1.1.1 255.255.255.0
serial restart-delay 0
interface Serial1/1
ip address 14.1.1.1 255.255.255.0
serial restart-delay 0
interface Serial1/2
ip address 15.1.1.1 255.255.255.0
serial restart-delay 0
interface Loopback0
ip address 1.1.1.1 255.255.255.0
router ospf 10
router-id 1.1.1.1
log-adjacency-changes
network 1.1.1.0 0.0.0.255 area 0
network 13.1.1.0 0.0.0.255 area 0
router bgp 1
no synchronization
bgp router-id 1.1.1.1
bgp log-neighbor-changes
neighbor liang peer-group
neighbor liang remote-as 2
neighbor liang ebgp-multihop 255
neighbor liang update-source Loopback0
neighbor 3.3.3.3 remote-as 1
neighbor 3.3.3.3 update-source Loopback0
neighbor 3.3.3.3 next-hop-self
neighbor 4.4.4.4 peer-group liang
neighbor 5.5.5.5 peer-group liang
no auto-summary
ip route 4.4.4.4 255.255.255.255 14.1.1.4
ip route 5.5.5.5 255.255.255.255 15.1.1.5
R2:
interface Loopback0
ip address 2.2.2.2 255.255.255.0
interface Serial1/0
ip address 24.1.1.2 255.255.255.0
serial restart-delay 0
interface Serial1/1
ip address 23.1.1.2 255.255.255.0
serial restart-delay 0
interface Serial1/2
no ip address
shutdown
serial restart-delay 0
inteface Serial1/3
ip address 25.1.1.2 255.255.255.0
serial restart-delay 0
router ospf 10
router-id 2.2.2.2
log-adjacency-changes
network 2.2.2.0 0.0.0.255 area 0
network 23.1.1.0 0.0.0.255 area 0
router bgp 1
no synchronization
bgp router-id 2.2.2.2
bgp log-neighbor-changes
neighbor liang peer-group 为了简化配置我们用peer-group
neighbor liang remote-as 2
neighbor liang ebgp-multihop 255
neighbor liang update-source Loopback0
neighbor 3.3.3.3 remote-as 1
neighbor 3.3.3.3 update-source Loopback0
neighbor 3.3.3.3 next-hop-self
neighbor 4.4.4.4 peer-group liang
neighbor 5.5.5.5 peer-group liang
no auto-summary
ip route 4.4.4.4 255.255.255.255 24.1.1.4
ip route 5.5.5.5 255.255.255.255 25.1.1.5
R3:
interface Loopback0
ip address 3.3.3.3 255.255.255.0
interface Serial1/0
ip address 13.1.1.3 255.255.255.0
serial restart-delay 0
interface Serial1/1
ip address 23.1.1.3 255.255.255.0
serial restart-delay 0
router ospf 10
router-id 3.3.3.3
log-adjacency-changes
network 3.3.3.0 0.0.0.255 area 0
network 13.1.1.0 0.0.0.255 area 0
network 23.1.1.0 0.0.0.255 area 0
router bgp 1
no synchronization
bgp router-id 3.3.3.3
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 1
neighbor 1.1.1.1 update-source Loopback0
neighbor 2.2.2.2 remote-as 1
neighbor 2.2.2.2 update-source Loopback0
no auto-summary
其他的配置都差不多不在贴出来了
我们看下BGP表
R1#show ip bgp
BGP table version is 3, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 6.6.6.0/24 4.4.4.4 0 2 i
*> 5.5.5.5 0 2 i
* 8.8.8.0/24 5.5.5.5 0 2 i
*> 4.4.4.4 0 2 i
R2#show ip bgp
BGP table version is 3, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 6.6.6.0/24 4.4.4.4 0 2 i
*> 5.5.5.5 0 2 i
* 8.8.8.0/24 5.5.5.5 0 2 i
*> 4.4.4.4 0 2 i
R3# show ip bgp
BGP table version is 9, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* i6.6.6.0/24 2.2.2.2 0 100 0 2 i
*>i 1.1.1.1 0 100 0 2 i
* i8.8.8.0/24 2.2.2.2 0 100 0 2 i
*>i 1.1.1.1 0 100 0 2 i
发现R3到6.6.6.0和8.8.8.0 下一跳都是R1
我们的要求是R3访问6.6.6.0/24 走R1 R4 访问8.8.8.0/24走R2 R5
第一种方式我们在R1 R2的out方向做
R1
access-list 1 permit 6.6.6.0
access-list 2 permit 8.8.8.0
route-map to-r3 permit 10
match ip address 1
set local-preference 400
route-map to-r3 permit 20
match ip address 2
set local-preference 300
route-map to-r3 permit 30
R1(config)#router bgp 1
R1(config-router)#neighbor 3.3.3.3 route-map to-r3
R2
access-list 1 permit 6.6.6.0
access-list 2 permit 8.8.8.0
route-map to-r3 permit 10
match ip address 1
set local-preference 300
route-map to-r3 permit 20
match ip address 2
set local-preference 400
R2(config)#router bgp 1
R2(config-router)#neighbor 3.3.3.3 route-map to-r3 out
我们需要的R3上软清下
R3#clear ip bgp * soft
R3# show ip bgp
BGP table version is 12, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* i6.6.6.0/24 2.2.2.2 0 300 0 2 i
*>i 1.1.1.1 0 400 0 2 i
*>i8.8.8.0/24 2.2.2.2 0 400 0 2 i
* i 1.1.1.1 0 300 0 2 i
发现第一跳确实变了，但我们看下R2 R1的BGP表
R1#show ip bgp
BGP table version is 3, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 6.6.6.0/24 4.4.4.4 0 2 i
*> 5.5.5.5 0 2 i
* 8.8.8.0/24 5.5.5.5 0 2 i
*> 4.4.4.4 0 2 i
R2#show ip bgp 直接走R4了 还不是我们想要的要求
BGP table version is 3, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 6.6.6.0/24 4.4.4.4 0 2 i
*> 5.5.5.5 0 2 i
* 8.8.8.0/24 5.5.5.5 0 2 i
*> 4.4.4.4 0 2 i
我们只是控制了R3到R1R2的选路
我们在R3 R4的in方向做
R1
access-list 1 permit 6.6.6.0
route-map liang permit 10
match ip address 1
set local-preference 500
route-map liang permit 20
neighbor 4.4.4.4 route-map liang in
R2
access-list 1 permit 8.8.8.0
route-map liang permit 10
match ip address 1
set local-preference 500
route-map liang permit 20
neighbor 5.5.5.5 route-map liang in
软清一下在R1 R2 R3
R1#show ip bgp
BGP table version is 4, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 6.6.6.0/24 4.4.4.4 500 0 2 i
* 5.5.5.5 0 2 i
* 8.8.8.0/24 5.5.5.5 0 2 i
*> 4.4.4.4 0 2 i
R2#show ip bgp
BGP table version is 4, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 6.6.6.0/24 4.4.4.4 0 2 i
*> 5.5.5.5 0 2 i
*> 8.8.8.0/24 5.5.5.5 500 0 2 i
* 4.4.4.4 0 2 i
R3# show ip bgp
BGP table version is 17, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* i6.6.6.0/24 2.2.2.2 0 100 0 2 i
*>i 1.1.1.1 0 500 0 2 i
*>i8.8.8.0/24 2.2.2.2 0 500 0 2 i
* i 1.1.1.1 0 100 0 2 i
我们的目的达到了！
总结：
loacl preference 有三种不同的方法进行配置
1.通过IGP路由引入BGP是关联route-map
2.针对IBGP peer 应用in and out 方向进行配置，对从peer收到或者通过的给peer的所有或部分进行设置
3.针对EBGP peer 应用in 方向的route-map针对peer接受到的所有或部分路由进行设置
本文转自q狼的诱惑 51CTO博客，原文链接：http://blog.51cto.com/liangrui/557941，如需转载请自行联系原作者
